The present invention relates generally to optical inspection systems, and more particularly to an automated photomask inspection apparatus for detecting or classifying defects that might occur on optical masks, reticles, and the like.
Integrated circuits are comprised of three-dimensional structures of conductors, semiconductors and insulators formed on a semiconductor substrate or wafer. The integrated circuit manufacturing process typically involves multiple processing steps, including repeated transferring of patterns to the wafer surface using photolithography. Precise and defect free execution of each step is essential for the production of functional semiconductor devices at a yield that is economical for the manufacturer.
Photolithography is used to transfer a pattern to a wafer by exposing a photoresist-covered wafer to a light intensity pattern and developing the photoresist. The light intensity pattern is produced from the modification of an incident light beam having an optical wave front, with a photomask or reticle. The photomask includes features having variations in optical properties that modify the amplitude and/or phase of the incident beam through reflection, transmission, absorption or any combination thereof. The light is then projected onto the wafer to produce a required intensity pattern. One type of photomask feature is an amplitude object that modifies the amplitude, or intensity, of an illumination beam. For a transmission photomask, an opaque layer of metal film is one example of an amplitude object.
A second type of feature is a phase object that modifies the phase of an illumination beam or phase distribution of the optical wave front thus producing desirable intensity distributions in the image projected on the wafer. A transparent layer of varying thickness on a transmission photomask is one example of a phase shift object (or xe2x80x9cphase shifterxe2x80x9d). A phase-shift mask (PSM) has a pattern of phase shifters on the mask that results in an illumination beam comprised of phase variations. In practice, a PSM may contain either phase objects or a combination of amplitude and phase objects, as well as attenuated phase objects that simultaneously induce a phase and amplitude variation at select wavelengths. For a transmitted-light PSM, phase objects can be variations in the thickness of the photomask, usually a fused quartz plate. As the light travels from the photomask to the wafer, interference occurs between the differently phase-shifted portions of the illumination beam. The system optics, including the illumination source and photomask, produce an interference pattern of light and dark areas on the wafer.
Semiconductor industry roadmaps call for decreasing the critical dimension (CD) in semiconductor wafers. Phase shift mask technology can enable smaller CD""s and is increasingly being used along with other approaches such as reducing the wavelength of the illumination beam or increasing the numerical aperture of the objective lens of the wafer stepper. Since most defects on a PSM will print, i.e. replicate itself, on every wafer in the particular semiconductor device, inspection for phase defects on PSMs is critical for achieving high yields. The industry is currently developing technologies to take advantage of the lower CDs of PSMs. These technologies include the ability to produce photomasks and the ancillary equipment and techniques for using and testing photomasks. In particular, most commercially available photomask inspection apparatus were designed before the widespread use of PSMs, as described for example, in U.S. Pat. Nos. 5,572,598 and 6,052,478, both to Wihl et al.
Of particular interest here is the inspection of masks that contain phase objects. Inspection of phase masks should include the detection and classification of the phase error, amplitude error of the phase shifter, amplitude error of adjoining features, and context. Since photomask inspection equipment usually performs an optical inspection at wavelengths and optical configurations different from that of the intended stepper, these differences must be taken into account.
The optical inspection of phase objects is technologically challenging. Pure phase objects modify the electromagnetic field of the incident light through changes in phase, and not of the intensity. As most detectors respond to variations in intensity, phase objects can be difficult to measure directly. Even more challenging is the detection of phase objects in the presence of amplitude objects. Due to the large amount of information contained on a mask, i.e. greater than 1015 pixels to write and greater than 1012 pixels to inspect a 6 inch mask, automated mask inspection systems are used.
A defect is defined here as any unintended modification to the intended photolithographic pattern caused during the manufacture of the photomask, or as a result of the use of the photomask. Defects can be due to, but not limited to, a portion of the opaque layer being absent from an area of the photolithographic pattern where it is intended to be present, a portion of the opaque layer being present in an area of the photolithographic pattern where it is not intended to be, chemical stains or residues from the photomask manufacturing processes which cause an unintended localized modification of the light transmission property of the photomask, particulate contaminates such as dust, resist flakes, cleaning residue, erosion of the photolithographic pattern due to electrostatic discharge, artifacts in the photomask substrate such as pits, scratches, and striations, and localized light transmission errors in the substrate or opaque layer.
Phase-shift photomask inspection systems should meet several requirements to be commercially successful. These requirements include the ability to: determine and report defects among about 1012 pixels in a reasonable time; inspect a single large die using gray-scale algorithms by rendering, from a database image of the PSM, both the transmission and phase for each pixel; handle multiple resolutions, including having a sensitivity to large (missing shifters) and small (sub-resolution) phase defects; process possible context information; account for off-wavelength inspection of phase errors; and distinguish between amplitude and phase objects. In addition to semiconductor applications, phase imaging systems are important in other fields, such as in the biological and medical areas.
Several imaging and defect detection systems for PSMs have been described in the prior art that address some of the above listed requirements. The following discussion provides information about several of the known techniques for obtaining phase information about objects useful in mask inspection systems.
Differential interference contrast techniques determine phase objects by interfering orthogonally polarized, laterally offset spots. Phase defects produce changes in the polarization state of the modified light that is detected as an intensity difference in the image, e.g. in a Nomarski microscope. Interference techniques have very high accuracy and can measure absolute phase. In practice however, interference techniques typically have very high alignment, optical wave-front quality, and system vibration requirements that are prohibitive or extremely costly to implement. Other problems include throughput and the need for a reference beam that could, in the case of the Nomarski technique, be blocked by chrome on the mask.
Differential phase contrast techniques, detailed in D. K. Hamilton and C. J. R. Sheppard, J. Microscopy 133, 27 (1984), determine the phase gradients of an object by the resulting intensity differences in the pupil plane of the imaging system measurable using a split or quad detector in the case of a scanner. Sensitivity of this technique depends on orientation of the phase object relative to the split detector that could result in missed defects.
The Zernike phase contrast technique uses a 90 degree shifted annular-pupil-plane filter to shift the 1st order against 0th order angular frequencies, making weak phase variations visible as intensity variations. For the case of an object with strong phase features, as in a phase shift mask, different pupil-plane filters may produce better sensitivity. However, Zernike-like imaging systems produce fringes in phase edge images, and are difficult to handle for database rendering in Die-to-Database (D:DB) inspection.
The defocused imaging approach generates an image using defocused optics. See, for example, C. J. R. Sheppard and T. Wilson, Phil. Trans. Roy. Soc. Lond. 295, 513 (1980), denoted as SandW in the remainder of this specification. Defocusing results in an imaginary transfer function that transforms phase variations to variations in amplitude. Prior art defocused imaging techniques for inspection of phase shift masks are not capable of handling arbitrary (i.e. non-repetitive), phase patterns, require a reference measurement to determine phase errors, or are limited to objects that can be resolved by the imaging system.
In aerial imaging, imaging parameters of the stepper are matched in the defect detection system, resulting in information on phase shifters. This is a low-resolution technique in which individual phase shifters are not resolved, and requires very high signal-to-noise ratios that are difficult and costly to implement in a high-speed imaging system.
Mach-Zehnder interferometry is an interferometric phase detection approach comparing the phase delay in the mask to the phase of external reference beam. As with other interference techniques, Mach-Zehnder techniques have alignment, optical wave front quality, and system vibration requirements that are prohibitive or extremely costly to implement.
While some problems in the detection of phase defects on a photomask have been addressed by the systems and methods disclosed above, there is still a great need to provide an efficient, highly sensitive phase defect detection system for a photomask that can be implemented at a reasonable cost. In particular, there is a need for a photomask inspection system that can determine the presence of phase defects, especially in the presence of amplitude objects, and that can be used to classify defects.
The present invention provides a novel method and system for optically measuring phase information in phase and amplitude objects that has many benefits over prior art techniques, particularly for the efficient inspection of photomasks or other articles having phase objects. In particular, many prior art limitations of phase object detection are surmounted with systems herein that inspect articles using one or more differently interacting illumination beams. In the present invention, the reflected or transmitted, or in general interacted or modified, beam(s) produce two or more responses that are different when illuminating the same area of the photomask or other articles that modify phase of the beam(s) depending on the presence of phase objects. The two or more responses are analyzed to provide two or more signals containing different phase information. The two or more responses may be produced simultaneously or sequentially from the interaction of the beam(s) with the article.
The present invention also provides methods and systems to inspect articles having phase objects by analyzing two or more signals containing different phase information generated by the interaction of the beam(s) with the objects in a manner that extracts the phase information from the amplitude information. The resulting system is thus sensitive to phase objects in the presence of amplitude objects, without greatly increasing the complexity of the optical system. For example, phase shift photomasks commonly contain amplitude objects, so that the present invention greatly simplifies detection of phase objects and phase defects on photomasks. In addition, the optical configuration of the present invention is particularly forgiving of variations in alignment and optical wave-front quality in comparison with prior art interferometric techniques. Also, the present invention is distinguished from other techniques in that it is capable of handling arbitrary (i.e. non-repetitive) phase patterns found in semiconductor photomasks.
In one embodiment, articles having phase objects are inspected by generating two or more signals resulting from the interaction and modification of illumination beams with the phase objects. The two signals are generated using systems, including but not limited to those having focus-based and Zernike-based optics, that are different for phase objects and amplitude objects, allowing for detection of one in the presence of the other using signal processing techniques. Another embodiment provides for a method and apparatus for classifying phase object defects using differently interacting beams. Comparison of information obtained using the interacted or modified beams provides information on the phase objects that can be used for inspection purposes, including, but not limited to the presence and classification of defects in a photomask or other articles in terms of defect size, phase error and context.
According to one particular embodiment of the invention, phase objects are inspected by supplying two illumination beams to an article that interacts differently according to variations in optical properties on the article. The different interactions can result from illumination beams that are differently focused or that have different Zernike point spread functions. The methods and systems of the present invention can be implemented using scanner-type or projector-type optical architectures, providing a variety of options for sequential, simultaneous, or multiplexed acquisition of the two signals or images. In addition, temporal, radial, or polarization separation can be used for creating and distinguishing the various signal or image channels in the latter two acquisition modes. For example, the different interaction of the two or more beams can result from the two beams having foci offset from one another that are used to inspect various locations on a photomask or other article. In illuminating the phase objects, these beams are differently focused (or defocused) from one another. Separation and measurement of the modified light provides sensitive information on the phase of objects at, and possibly near, the measurement location.
In one implementation of the above embodiment, a single focused illumination beam is used whose position is varied, relative to the photomask or other article, between two differently focused positions to obtain two signals of modified illumination beam intensity. Multiple scanning beams may be produced from a single beam from a radiation source. A birefringent lens and objective lens may be used to focus each of the scanning beams into pairs of differently focused traveling beams, which are separated after modification by the photomask. The individual modified beams may be separated according to characteristics of the individual beams, such as by their polarizations. An alternative implementation uses a multifocal zone plate at the system pupil and an objective lens to produce pairs of differently focused beams for each input beam, followed by spatial filtering to separate the individual modified illumination beams.
According to yet another particular implementation of the above embodiment, one or more illumination beams are converted to pairs of illumination beams slightly defocused relative to each other. The resulting pair of illumination beams is focused onto the same approximate area of the article. Illumination beams modified by the article are separated, producing signals representative of each modified illumination beam, and the individual pairs of signals are compared to provide phase information. The pairs of illumination beams may be scanned across the photomask to produce a map of the phase difference induced by the illumination beams. The comparison of pairs of differently focused signals provides information is less sensitive to amplitude objects than that provided by other phase detection techniques, and is capable of detecting phase differences of 30 degrees or less. In addition, three other implementations alternatively call for 1) using a single illumination beam, 2) one pair of illumination beams, or 3) multiple pairs of illumination beams. Thus the novel features of the present invention may be implemented using a variety of optical systems for focusing a beam at approximately the same position on a photomask or other article, but at different depths through the article.
In another particular embodiment of the present invention, photomasks are inspected using two or more beams results from beams having different Zernike point spread functions. Thus, for example, coaxial beams having complementary plus and minus 90 degree point spread functions are modified by an article according to phase objects thereon. The different point spread functions may be introduced, for example, by a Zernike plate at the back focal plane of an objective, where such introduction can occur prior to or after interaction of the beams with the photomask. The two images of the article produced thereby can be separated and compared according to their corresponding locations. Inspection using complementary point spread functions (such as by using Zernike plates) provides increased sensitivity to phase objects over prior art Zernike methods.
It is another aspect of the present invention to provide a method and apparatus for detecting phase object defects in photomasks by comparing one defocused image with a database of expected defocused image data. Comparison of the interacted beam with the database provides information on the phase objects that can be used for inspection purposes, including, but not limited to the presence and classification of defects in a photomask.
A further understanding of the invention follows from the detailed discussion of specific embodiments below. This discussion refers to devices, methods, and concepts in terms of specific examples. However, the method of the present invention may operate with a wide variety of devices. It is therefore intended that the invention not be limited by the discussion of specific embodiments.
All publications and patents cited herein are hereby incorporated by reference in their entirety for all purposes. Additional objects, advantages, aspects and features of the present invention will become apparent from the description of preferred embodiments, set forth below, which should be taken in conjunction with the accompanying drawings, a brief description of which follows.